SEOUL — North Korea appears to be readying for a possible third nuclear test as early as next March, a newspaper reported Wednesday, as New Mexico Gov. Bill Richardson headed to Pyongyang on an unofficial diplomatic foray.

U.S. and South Korean intelligence have been watching the North's nuclear sites for any activity.

Analysts say the North could use a test to try to gain leverage in international talks it is seeking and secure aid to prop up its destitute economy.

The New York Times reported Wednesday that the Obama administration has concluded that that North Korea's new plant to enrich nuclear fuel uses technology that is "significantly more advanced" than what Iran has assembled.

North Korea already has the fuel for as many as 12 nuclear weapons, the newspaper said.

South Korea's Chosun Ilbo daily on Wednesday cited an intelligence official from Seoul as saying a tunnel was being dug at the country's nuclear test site that could be completed in March next year, possibly heralding a new nuclear test.

South Korea's foreign ministry said it was closely monitoring the site and said there was no concrete evidence to show the North Koreans were preparing for a third test.

The amount of earth removed from the site in Punggye township, in a northeastern region of North Korea, indicated the tunnel was about 550 yards deep, half the depth needed for a nuclear test, the Chosun Ilbo report said.

"North Korea is digging the ground pretty hard ... at its two major nuclear facilities," a South Korean intelligence official was quoted as saying.

North Korea is also speeding up work on new construction at its main Yongbyon nuclear complex, where it revealed a previously unknown uranium enrichment facility last month, the newspaper quoted intelligence sources as saying.

South Korea's foreign ministry declined to confirm the details of the report, but said: "Nothing has been confirmed that would prove the North is preparing to conduct a nuclear test."

The South's nuclear envoy, Wi Sung-lac, was in Moscow meeting with his Russian counterpart, in the same week that Russian Foreign Minister Sergei Lavrov met North Korean Foreign Minister Pak Ui-chun and chided him over the North's nuclear program.

Civil defense drill
The report coincided on Wednesday with South Korea's largest civil defense drill in recent years after North Korea shelled an island near their disputed maritime border, killing four people, last month.

The exercise on a busy week day brought traffic to a standstill nationwide and saw mass evacuations to bomb shelters.

Prime Minister Kim Hwang-shik said the aim was to heighten readiness for a possible North Korean air raid and he also warned the North to expect reprisals if there was another attack.

"The government is ready to demonstrate that there will be due price to pay for any future aggression," he said.

Analysts say North Korea's unveiling of a modern uranium enrichment facility and preparations for another nuclear test were likely to be ploys to pull regional powers back to the negotiating table, where Pyongyang hopes to secure aid.

The impoverished state has in the past won economic assistance and diplomatic attention at six-country talks aimed at persuading it to abandon its nuclear ambitions.

Deputy U.S. Secretary of State James Steinberg was in Beijing to consult with the Chinese on North Korea and Richardson was on his way to the North.

"My objective is to try to get North Korea to calm down a bit, see if we can reduce tension in the Korean peninsula," Richardson said as he arrived at an Albuquerque airport for the first leg of a commercial flight to Beijing and then to North Korea.

The governor will return to New Mexico on Dec. 20.

This will be Richardson's third trip to North Korea since he became governor in 2003.

Richardson served as U.N. ambassador in the Clinton administration and often has played the role of an unofficial diplomatic troubleshooter, including missions to secure the release of hostages in Sudan, Iraq and North Korea.

The State Department has said Richardson isn't delivering a message to North Korea for the U.S. government. However, Richardson's contacts with North Korean officials may provide insights for diplomats trying to persuade North Korea to abandon its nuclear weapons program.

North Korea showed a uranium enrichment facility, which could give it a second route to make nuclear bombs, at the Yongbyon site to a U.S. expert in November and later announced it was operating such a program under a "peaceful" energy project.

South Korea's foreign minister said on Tuesday he suspected there were more facilities in addition to Yongbyon where the North was enriching uranium. A media report said Pyongyang had three to four such plants.

North Korea conducted nuclear tests at the Punggye site in 2006 and 2009, when detonations in tunnels were detected by U.S. and South Korean monitoring.

The U.N. Security Council condemned last year's test and imposed tough sanctions aimed at banning North Korea's arms trade and cutting off funding for such programs.

Analysts say ailing leader Kim Jong-il's plan to transfer power to his son Jong-un is also creating domestic political pressure, as the regime resorts to military grandstanding to try to build legitimacy for the untested and previously unknown successor.

The Associated Press, Reuters and msnbc.com staff contributed to this report.

Video: Photos show possible advances in N. Korea nuclear program

>>>have heard this
north korea
threatened the south with
nuclear war
yesterday and, of course, that raises the question, does the north have
nuclear weapons
? in an exclusive interview with the scientific observer who was shocked add what he found in
north korea
last month,
richard lui
gets his personal views and also saw some pictures that few have seen. dr. siegfried heckert first trip to
north korea
was
2004
.

>>i actually wound up in a
conference room
in the reprocessing acilitfacility.

>> reporter: six more times he made that trip, taking his own pictorial log. after number seven, he says what nobody wants to hear,
north korea
can now help create new north koreas. nations with nuclear capability that wouldn't hesitate to use it against the u.s. iran could be one of them. first because
north korea
despite under sanctions is able to export millions of dollars of arms annually says this u.s. report. as a document from wikileaks shows, that could include missile technology and then november's shocking new find.

>>the
north korean
technology that i saw is ahead of iran's.

>> reporter: last year he snapped this picture. this year he sees 2,000 as he calls it, beautiful centrifuges as his mock up shows here.

>>it was really quite stunning to see that because i simply didn't expect them to have this sophistication and this scale of facility.

>> reporter: and they built it in one year, almost impossible to do. this is the nuclear site in
2009
and then in
2010
. but there could be more.

>>there's another facility of some size still operating.

>> reporter: perhaps making bomb fuel, he says.

>>the past facilities i've been in, the equipment, especially the control equipment is old style, sort of '50s america style.

>> reporter: but the new
control room
--

>>what you would see in a good facility today in the
united states
.

>> reporter: he talks openly about what he sees.

>>the
north koreans
expect me to do that because that's the way they can actually have an effect.

>> reporter: an effect on a country that hecker says enough plutonium for four to eight nagasaki-sized bombs.

>>the message clearly was, look, we have the plutonium and if we have the plutonium, that means zee the ba s they have the bomb. they wanted me to take that message back to the
u.s. government
and they say
north korea
has the bomb and they want some respect.

>> reporter: some ask how we can give respect when you see pictures of hungry
north koreans
. but despite being under sanctions he sees what a
poor country
could afford. like this new uranium facility with its blue roof. instead boldly saying, we are here.

>>richard lui
is here now. some very sobering comments from the scientist and you also got to see quite a few photographs that he took, right?

>>dr. hecker long the way in his last seven visits have snapped some pictures. basically what he is asking in one way sanctions have been happening in recent times, but are they working? let me show you this picture first. this was of a tower that was being built over the course of 20 years.
2009
he and he comes back in
2010
, look at that. done over the last year. we can zoom in closer to see some of the work that was not done and then it was done over the course of one year. another picture for you,
2009
. on the left-hand side i want to show you this, chris. this
phone booths
, not superman type but
phone booths
and they have cell phones. not a
big deal
. what a critic will ask, are sanctions working because they're still able to develop their nuclear functionality and the enrichment process as well as their own economy. in fact, a u.n. report that said that
international trade
of some $7 billion in
2009
, that is double from the year before. so, the question is, what is the solution to
north korea
based on its new capability to potentially create more north kore koreas. we care about that here in the
united states
.

A South Korean border guard mans a post through a fence draped with re-unification ribbons near the border village of Panmunjom in Paju, South Korea, on Dec. 22, 2010. South Korea vowed Wednesday to "punish the enemy" as hundreds of troops, fighter jets, tanks and attack helicopters prepared massive new drills near the heavily armed border a month after a deadly North Korean artillery attack.
(Wally Santana / AP)
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A North Korean defector takes part in a candle light vigil on the eve of the one month anniversary of the North Korea's attack on Yeonpyeong Island in downtown Seoul, South Korea, on Dec. 22.
(Ng Han Guan / AP)
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A combination of photos shows North Korean soldiers taking part in a shooting exercise at a field in Kaepoong county, on the north side of the demilitarized zone (DMZ) separating the two Koreas, in this picture taken from south of the DMZ in Paju, about 31 miles north of Seoul, on Dec. 22.
(Jo Yong-hak / Reuters)
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Bae Bok-soon (R), an older sister of Bae Bok-chul, cries during the funeral for the two civilians who died when North Korea shelled Yeonpyeong Island on November 23, in Incheon, west of Seoul on De. 6.
(Jo Yong-hak / Reuters)
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Crew members watch as an F/A-18E Super Hornet lands on the deck of the aircraft carrier USS George Washington during a naval exercise with South Korea in the Yellow Sea on Tuesday, Nov. 30. The drills come amid heightened tension in the region after a North Korean artillery attack on South Korea's Yeonpyeong island last week.
(Park Ji-hwan / AFP - Getty Images)
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Staff watch radar screens in the Combat Direction Center on the USS George Washington during the military drills off South Korea.
(Wally Santana / AP)
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Former South Korean special agents whose mission was to infiltrate North Korea, sing a military song during a rally on the Yeonpyeong island, South Korea. About 85 former agents, who criticized the North's attack and urged the South Korean government to punish Pyongyang, landed the island Nov. 30 and said they would stay for a week to help with reconstruction.
(Lee Jin-man / AP)
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South Korean marines watch President Lee Myung-Bak's news conference on a television minitor on Yeonpyeong island on Nov. 29. Lee condemned North Korea's recent shelling of the South Korean border island, calling an attack against civilians an "inhumane" crime.
(Jeon Heon-Kyun / EPA)
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South Korean ships stage off the coast of South Korea's Yeonpyeong Island on Nov. 28 as war drills by the United States and South Korea began.
(David Guttenfelder / AP)
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South Korean protesters hold candles during a rally in Seoul opposing the military exercise between South Korea and the United States.
(Park Ji-hwan / AFP - Getty Images)
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South Korean women take cover inside a bomb shelter on Yeonpyeong Island after authorities sounded the alarm over a possible North Korean rocket attack on Nov. 28. It proved to be a false alarm.
(David Guttenfelder / AP)
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A North Korean soldier, right, looks back as she and another soldier patrol on a pathway along the bank of the Yalu River near Sinuiju, North Korea, Nov. 28.
(Andy Wong / AP)
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A South Korean police car is reflected in the shattered glass of a restaurant window along a seaside road on Yeonpyeong island on Nov. 27.
(David Guttenfelder / AP)
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Former South Korean marines burn images of North Korean leader Kim Jong Il, right, and his son Kim Jong Un, during a rally Nov. 27 in Seoul.
(Wally Santana / AP)
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Kim Oh-bock, mother of Seo Jung-woo, a South Korean marine killed in the Nov. 23 North Korean bombardment, cries as she holds his casket during a funeral service Nov. 27 at a military hospital in Seongnam.
(Ahn Young-joon / AP)
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South Korean protesters denouncing North Korean attack on an island close to the border between the two nations burn a North Korean flag in Seoul on Nov. 24. After North Korea's strike, South Korea and the United States said they would launch four-day naval exercises in the Yellow Sea involving an American aircraft carrier.
(Jung Yeon-Je / AFP - Getty Images)
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A picture taken off television shows the moment of impact of one of the artillery shells fired by North Korea onto the South Korean island of Yeonpyeong.
(Reuters TV)
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Related video

This picture taken by a South Korean tourist shows
huge plumes of smoke rising from Yeonpyeong Island in the disputed waters of the Yellow Sea on Nov. 23. North Korea fired dozens of artillery shells onto the South Korean island, killing two people, setting homes ablaze and triggering retaliatory fire by the South. It was one of the most serious clashes between the two sides in decades.
(AFP - Getty Images)
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Editor's note:
This image contains graphic content that some viewers may find disturbing.